1. Field of the Invention
This invention generally relates to a microwave oven, and more particularly a wave guide system of a microwave oven, which guide microwaves generated by a magnetron into a cavity of the oven through microwave feed openings of the wall of the cavity to heat food products placed in the cavity.
2. Description of the Prior Art
Generally, a microwave oven is a cooker for dielectrically heating food products placed in a cavity by microwaves which are generated by a magnetron, directed to a waveguide, and then emitted into the cavity to be incident upon the food products. Microwave ovens are classified into a single feed type, a dual feed type and a multiple feed type depending upon the number of a microwave feed opening for feeding the microwaves into the cavity.
One of the conventional microwave ovens of a single feed type is of the type as shown in FIGS. 1a and 1b of the accompanying drawings, which comprises a cavity 301 having an internal space and a microwave feed opening 302 formed through one side wall thereof the feed microwaves into the cavity; a tray 305 disposed centrally of the bottom of the cavity 301 to support thereon a loading, i.e., a food product and rotatably driven by a motor 306; a waveguide 303 disposed externally of the cavity 301 to surround the microwave feed opening 302 and be in communication with the feed opening; and a magnetron 304 mounted on the back of the waveguide 303.
With this construction, when electric power is applied to the microwave oven, the magnetron 304 generates microwaves, which are in turn introduced into the waveguide 303, and then emitted into the cavity 301 through the single microwave feed opening 302 of the side wall of the cavity to be incident upon the food on the rotating tray 305, thereby effecting cooking of the food through dielectrical heating action.
This prior microwave oven having the single microwave feed opening through which the microwaves pass to be directed to a food product in the cavity however has drawbacks as follows:
First, in cooking of planar, i.e., substantially flat, food products such as, for example, a laver (a kind of seaweed), a squid, a pizza and the like, uniform heating may not be achieved, so that the central portion of the food may be over-heated to be burnt black. Second, when heating food products contained in containers of a given height, such as a milk bottle, cup and the like, there may occur a phenomenon in which the upper portion of the container is more concentratively heated than the lower portion so that there exist a heating temperature difference between the upper and lower portions of the container. As a result, the heated liquid beverage such as, for example, milk, tea, Chinese medicine, or the like can give a feeling of discomfort to an eater due to the temperature difference between the upper and lower areas of the liquid beverage in the container. Third, since a food product is not heated uniformly as a whole, it is necessary to extend the cooking time in order to additionally cook the insufficiently heated portion, resulting in increasing power consumption.
Accordingly, with a view to overcoming the problems caused by the microwave oven of a single feed type as described above, there have been proposed microwave ovens of a dual feed type having two microwave feed openings formed in a side wall of a cavity, typical examples of which are disclosed in U.S. Pat. No. 5,057,660 and European Patent Publication No. 0,478,053.
The prior art microwave ovens of a dual feed type as disclosed in the above mentioned patents will now be summarized with reference to FIGS. 2a, 2b, 3a, and 3b of the accompanying drawings.
First, referring to FIGS. 2a and 2b showing longitudinal cross-sectional and exploded fragmentary perspective views of the microwave oven of a dual feed type as disclosed in U.S. Pat. No. 5,057,660, the oven comprises a cavity 201 having a pair of upper and lower microwave feed openings 206a, 206b formed in one side wall thereof; a pair of upper and lower heaters 202, 202 disposed in the cavity 201; a plurality of pairs of racks 204 formed at different spacings on the opposite side wall surfaces of the cavity 201 to allow a shelf 203 to be adjustable supported at selected height depending upon the size of a loading or a food to be placed on the shelf; and a planar cover plate 209 attached to the outer wall surface of the cavity 201 opposed to the racks 204 between the upper and lower feed openings 206a, 206b to permit easy production of standing waves in a waveguide 205.
The waveguide 205 for guiding microwaves generated by a magnetron 207 is mounted on the outer wall surface of the cavity 201 to cover all of the microwave feed openings 206a, 206b and cover plate 209, and the magnetron 207 is mounted on the outer surface of one wall of the waveguide 205 with a protruding antenna 208 thereof inserted into the waveguide. This microwave oven is usually referred to as a multifunctional microwave oven having both an electric heater heating function and a microwave heating function.
In operation of the microwave oven thus constructed, when it is desired to cook a food product by using the heaters, a heater mode is selected, and then electric power is applied to the oven in the state in which the food product is placed on the shelf 203 supported by the racks 204. As a result, the upper and lower heaters 202, 202 disposed in the cavity 201 are energized to heat the food product.
Then, when it is desired to cook a food product by using microwaves, a microwave mode is selected, and then electric power is applied to the oven. As a result, the magnetron 207 generates microwaves, which are in turn emitted upon the food in the cavity 201 through the upper and lower microwave feed openings 206a, 206b to dielectrically heat the food.
This microwave oven of a dual feed type is however disadvantageous in that since the waveguide 205 becomes longer and the separate cover plate 209 must be attached to the outer surface of the wall of the cavity 201 to permit the production of the standing waves in the waveguide 205, material costs and the number of manufacturing processes are increased, resulting in higher manufacturing cost. In addition, since a short circuited surface is not provided between the antenna 208 of the magnetron 207 and one side surface of the waveguide 205, the standing waves are not sufficiently produced in the waveguide, so that the output and uniform heating performance of the oven may be lowered.
Then, referring to FIGS. 3a and 3b showing longitudinal cross-sectional and schematic perspective views of the microwave oven as disclosed in European Patent Publication No. 0,478,053, the oven comprises a vertically extending waveguide 105 formed integrally with one side wall of a cavity 101 and having a protruding portion 104 formed at the upper portion of its outer wall and upper and lower microwave feed openings 106a, 106b formed in the upper and lower portions of its inner wall to be in communication with the interior of the cavity; and a magnetron 107 having an antenna 108 and mounted on the protruding portion 104 of the waveguide 105 with the antenna inserted into the protruding portion in spaced apart relation to the portion to form therebetween a short circuited surface. The width of the protruding portion 104 of the waveguide is chosen to be substantially equal to the length of the antenna 108, and the waveguide 105 is provided with a horizontal top wall 105a and an inclined lower wall 105b. Further, the distance between the upper and lower microwave feed openings 106a, 106b is chosen to become as great as possible such that the upper and lower feed openings are located near to the upper and lower ends of the waveguide 105, respectively.
With this construction, when the microwave oven is turned on, the microwaves generated by the magnetron 107 and emitted through the antenna 108 produce standing waves in the waveguide 105 through the protruding portion 104 of the waveguide short-circuited with the antenna, and then are in part emitted directly into the cavity 101 through the upper microwave feed opening 106a of the waveguide, while the remainder are reflected from the inclined lower wall 105b of the waveguide, and then emitted into the cavity through the lower microwave feed opening 106b. As a result, an interference field is formed in the cavity so that the food in the cavity can be heated uniformly.
However, this prior art microwave oven also has drawbacks in that since the waveguide 105 becomes longer to provide the great distance between the upper and lower microwave feed openings 106a, 106b and the protruding portion 104 must be additionally provided on the outer wall of the waveguide to form the short circuited surface for producing the standing waves in the waveguide, the waveguide is relatively complex in construction, resulting in increasing the number of manufacturing processes, and hence manufacturing cost. Further, the long waveguide leads to difficulty in arranging the parts of an electric apparatus in a space below the waveguide adjacent one side wall of the cavity during assembling operation.